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Formatting (14 of 14)

Overview

The default way in which APL displays results may not always suit your requirements. Obviously you can do a certain amount by using functions like size to reshape data, or catenate to join data items, but for many applications you may want much more sophisticated facilities.

You may, for example, want to insert currency signs and spaces in numeric output, or produce a neatly formatted financial report, or specify precisely the format in which numbers are displayed.

APL includes flexible ways of formatting data. Formatting functions both:

• convert the format of data from numbers to characters
• allow you to specify how (converted) numeric data will be laid out.

The formatting functions available will depend on which APL interpreter you are using, so the following is only a guide. You will need to check your APL's documentation for available formatting functions.

Here we give details of two common functions: ⍕ (Format or Thorn) and the system function ⎕FMT available in every APL but APL2. They can be used purely to convert numeric data to characters. The converted data looks the same, but has the properties associated with character data.

Additionally, both functions let you specify how many character positions a number should occupy when it's displayed, and how many of these positions are available for decimal places. The number of characters and number of decimal places are specified in the left argument.

The "Format" primitive

An example without left argument:

      ⍕ 0.0000003  3.0123456789
3E¯7 3.012345679

(Note that the result depends on the system variable ⎕PP = Print Precision)

      6 2 ⍕ 341.82921
341.83

(Note that since the number had to be truncated to fit the character positions allowed, it was first rounded to make the truncated representation as accurate as possible.)

The system function ⎕FMT

Note that ⎕FMT is not available in APL2.

⎕FMT takes the process a stage further, allowing a variety of picture phrases, qualifiers and decorators to be supplied as the format specification.

      'B K2 G< ZZ9 DOLLARS AND 99 CENTS>' ⎕FMT 8.23 12.86 0 2.52
   8 DOLLARS AND 23 CENTS
  12 DOLLARS AND 86 CENTS

   2 DOLLARS AND 52 CENTS

⎕FMT is a very powerful function which allows you to format numbers in all sorts of ways. Not surprisingly it is one of the most complex functions available in modern APLs.

End of Tutorial

That's the end of the APL Tutorial. Congratulations on making it all the way to the end! If you're still hungry for more, you might want to check out Further Topics in APL which covers some more advanced topics.

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